1. Field of the Invention
The present invention relates to a method for making overprint predictions.
2. Discussion
In prior art it is known that in package printing for example, a great variety of special colors are used which are combined with each other during the printing process by overprinting. From a library of thousands of special colors, a few colors to be combined are selected, for example 8 to 12 colors. In practice, this produces a great number of color sets which are different from application to application. From each such color set intermediate hues are produced by creating hue gradations using for instance a screening technique and printing these hues gradually one upon the other. Thus many combinations both of intermediate hues and color sets are totally possible.
There are different reasons for an existing need of information on the appearance of an overprint. This can be used for instance in making proof prints for true-color displaying on the monitor, for the determination of color combinations which are more favorable while having the same color effect, and so on. All this information usually relies on the CIELAB value of a color combination in printing, this value being a standardized value based upon a degree of spectral reflectance (L*a*b* color space of CIE, CIE publication 15-2004).
Considering that the appearance is also dependent on the material of the substrate, an overprint prediction could be made with highest accuracy only if all color sets with their colors in all shades and all overprint combinations were applied to the respective substrate and evaluated. Such efforts would not be economical. Nevertheless, it is one of the methods known in prior art in which hundreds of color combinations and even more have to be printed as color fields which are subsequently tabulated using measuring values obtained by spectral analysis. This measurement only determines the degree of spectral reflectance of each field as a whole, hence the combined effect of the contribution of each color component and of the printing substrate on the passage of light. Although not all overprint combinations are required, the reduction to only a few shades per individual color must be increasingly broad with an increasing volume of the color set. Including printing and evaluation, the effort for each color set is high.
A modification of such a prior art method provides for shades of the individual colors to be printed on a partly pre-treated printing substrate, i.e. on a blank substrate or on a substrate that is pre-printed in black and grey for example, the shades being used to determine the specific characteristics of the respective printing color. This method is also complicated and in particular it is comparatively inaccurate concerning the prediction as to the effect of overprinted colors.
The pre-known systems are described for example in WO 98/46008 or US 2005/0094209.
Within an improvement of the known method, US 2006/0007252 describes the use of the so-called Neugebauer model for overprint calculation. Here the spectra of the individual full tones on the one side and of the full tone overprints on the other side are used. A correspondingly improved system is disclosed in U.S. Pat. No. 7,423,778. However, the method there described still necessarily requires the measurement of several overprints, namely with respect to full tones on full tones and with respect to intermediate tones on full tones.